Mechanically entrained fluidic oscillator



Sept. 3, 1968 w. J. WESTERMAN. JR

MECHANICALLY ENTRAINED FLUIDIC OSCILLATOR Filed April 1, 1985 2 Sheets-Sheet 1 m Mm Du m V 0 2 r nu G m L Ill D. Du. rr. m m m n Du rr. mm S w S F. m NN E mo V F. 2 El! B u. \I W m f w ES HH rr nU M 2 T MWC E I U. 0 R V A P W R AM o o m w mm l S on To 7. NM Emm fi C V Il MT AH I| E 0 EN LW R C0 F. EG. N IA JN 0 LR I .4 M 8 P 0| .HT P M s VA VA 0 ME X V L U N TORS|0NAL MEMBER INVENIOR WILLIAM J. WESTERMAN,JR.

Se t. 3, 1968 w. J. WSTERMAN. JR 3,

ECIANICALLY ENTRAINED FLUIDIC OSCIL.ATOR

2 Sheets-Shet 2 Filed April 1, 1965 INVENTOR WLLIAM J. WESTERMAN. JR.

ATTORNEY nited States Patent 3,399,688 MECHANICALLY ENTRAINED FLUIDIC OSCILLATOR William J. Westerman, Jr., Winter Park, Fla., assignor to Martn-Marietta Corporation, Middle River, Md., a corporation of Maryland Filed Apr. 1, 1965, Ser. No. 444,561 4 Claims. (Cl. 13781.5)

ABSTRACT OF THE DISCLOSURE This invention relates to a mechanically entrained fluidic oscillator having a very stable frequency and utilizng a plurality of fluidic components. The physical arrangement includes an oscillatory member, a bistable fluid element for powering the oscillatory member, and a pair of monostable fluid elements operated by said oscillatory member, arranged to cause the switching of said bistable element at the correct time to assure a very stable frequency. Switching occurs in response to the time at which the oscillatory member interrupts a fluid flow to the control port of a monostable member. Advantageously, no close manufacturing tolerances are involved in the construction of this oscillator.

This invention relates to fluid per-ated devices and more particularly to fluidic oscillators having a precise operating frequency.

The growing interest of recent years in fluid analogs of electronic components has led to the development of amplification, logic, control and computer systems employing pressurized fluid as the basic energy medium. In such systems the elemental control components typically incorporate a power nozzle from which a deflectible welldefined stream or jet of pressurized fluid issues and one or more control uozzles or orifices capable of influencing the direction of deflection of the power jet. The degree of deflection of the power jet may be proportional to the influence exerted on it by the control nozzle or orifices. Alternately, the configuration of the fluidic components may be designed to provide a digital characteristc whereby the power jet is controllably deflected between discreto angular output positions.

Digital fluidic components often make use of the C0- anda effect, that is to say, of the propensity of a fluid jet to attach itself under certain conditions to a surface adjacent the jet and to flow along that surface until deflected therefrom by an additional influence. This invention is concerned with the use of the Coanda effect in fluidic components having digital characteristics.

In fluidic logic or computation systems it often becomes important to make time computations or measurements. This imples the use of an oscillator. Fluidic oscillators have been proposed in which the power jet is switched back and forth between two angular orientations. One type of fluidic oscillator has been proposed in which the frequency of oscillation is determined in part by the length of a return path connecting two control orifices on opposite sides of the power nozzle. The Warren Patent No. 3,016,066 is illustrative of this type of device. High pressure Waves travelling at sonic velocities back and forth between the two control orifices force the power jet to switch its angular position alternately depending on the time of arrival of the pressure wave at the control orifices. Unfortunately, the frequency of such devices is temperature sensitive because the sonic velocity in a gas is proportional to the square root of the absolute temperature of the gas. Oscillators such as these therefore function more effectively as digital thermometers than as time measurement devices.

Other types of fluidic oscillators have also been pro posed, but these have almost invariably been either temperature sensitive, or pressure sensitive. Some interesting types of devices have been built which are both pressure-sensitive and temperature-sensitive but which make use of this dual sensitivity to effect a compensation. Thus, when either the temperature or the pressure increases the frequency of oscillation of such devices tends to increase. Compensation mechanisms have been provided which bleed off a portion of the gas in the system thus lowering the supply pressure as the temperature rises in an attempt to maintain a constant frequency. Unfortunately, the pressure bleed is an open-loop device and as such must be made With a very high degree of accuracy. Even when this is done with great precision, the frequency stability of the oscillator falls far short of expectations for use in precision logic or computation systems.

The present invention has as an object to provide fluidic oscillators having an accurately determined frequency of operation even though the temperature or pressure of the fluid employed in the oscillator may vary widely.

A further object of the invention is to provide fluidic oscillators having a mechanical vibrator introduced within a feedback path of a monostable or bistable fluidic oscillator to entran the oscillator at a constant frequency.

By way of a brief summary of one permissible embodiment of the invention, a fluidic oscillator is provided in which a high pressure well-defined stream or jet of fluid issues from a power nozzle between two diverging surfaces. The jet can assume either of two angular orienta tions flowing along one surface or the other. However, the geometry of the device is such that, due to Coanda effect, the jet favors a preferred monostable orientation along one of the walls and the jet is stable in this position. A switching orifice on the opposite side of the power jet is normally closed by an oscillatory member, such as a tuned vibratable member. As the gas in the switching orifice is entrained by the power jet, creating a low pressure condition at the switching orifice, the power jet is pulled away from its preferred orientation and directed along a feedback path to impinge on the vibrata-ble member. The fluid in the feedback path exctes the vibrator to oscillate or vibrate at its natural frequency, In vibrating the vibrator perodically opens the switching orifice and permits the power jet to return to its monostable orientation. Switching of the jet takes place cyclically at the frequency of the vibrator. When the frequency of the tuned vibrator is adjusted in dependence on the length of the feedback path, a very high degree of stability of frequency results.

Other embodiments can involve a torsionally restrained disc capable of vibrating with a rotational oscillation, as well as an embodiment involving the use of both monostable and bistable elements in concert.

Although the scope of the invention is not to be limited except by a fair interpretation of the claims appended hereto, further details of the invention as well as additional objects and advantages of the invention in different embodiments will be more readily understood in connection with the following more detailed description taken together with the accompanying drawings wherein:

FIGURE 1 is a partially pictorial, partially diagrammatic view of a fluidic oscillator constructed in accordance With the principles of the this invention;

FIGURE 2 is a graph representing certain time relationshps involved in the operation of the oscillator shown in FIGURE l;

FIGURE 3 is a perspective view of an alternate form of oscillating frequency control element for use in the practice of this invention; and

FIGURE 4 is an isometric view of an embodiment of my mechanically entrained oscillator, wit-h portions removed for clarity, in which a bistable element, as well as two monostable elements are employed.

If two linear oscillators are coupled together, a beat frequency results that is a function of the frequency difference of the two oscillators. As the difference in frequency decreases so does the beat frequency. If, on the other hand, two nonlinear oscillators are coupled together the coupling causes both to oscillate at a common frequency and to lock on this frequency whenever the frequency differences are small. This phenomena is known as frequency entrainment. The zone of entrainment is that range of frequences in which the lock-in may be expected to occur. If the two oscillators are of almost the same natural frequency, the zone of entrainment becomes vanishingly small.

ln a preferred embodiment this invention makes use of the phenomenon of frequency entrainment to stabilize the frequency of a fluidic oscillator. Unless one of the oscillators has a stable natural frequency, it is unlikely a constant operating frequency will result Thus instead of attempting to sta-bilize a free running fiuidic oscillator, this invention contemplates entraining or exciting a nonlinear fluidic oscillator with a relatively low power resonator having a stable reference frequency. The device shown in FIGURE l is a monostable fiuidic oscillator whose frequency of operation is entrained by a mechanical vibrator in such a manner that its resonant frequency is substantially unaffected by temperature changes, variations in fiuid pressure, or translational accelerations.

In this embodiment of the invention the cover plate normally attached to the top of the device has been removed, exposing the interior configuration of the oscillator. The body of the device may be of any solid material and may even be formed of molded plastic material. Those familiar with fluidic devices in general will recognize the high pressure fiuid chamber to which is supplied a pressurized fluid such as air or gaseous combustion products from a suitable supply source not illustrated- Chamber 10 functions as a flud source for power nozzle 11 from which a jet of fluid issues passing between a pair of orifices 12 and 13 into an exit chamber 14 between two divergng surfaces 15 and 16. An angular apex 17 facing the power nozzle 11 divides the chamber 14 into two exit channels. The first of these channels 20 adjacent surface 15 may lead to a utilization device sensitive to the state of the oscillator. The other of these channels 21, which may be termed a feedback channel is curved to direct the fluid therein toward a vibratable member shown generally at 22.

The vibratable member 22 is shown somewhat generally in this illustration to represent a member which may be anchored under tension at opposite ends and which may be set into vibration by forces acting thereon. The vibrator 22 may of course be tuned to a predetermined frequency, as by varying the tension therein or by altering the vibratable mass thereof. It is to be understood that many forms of vibratable members may be employed in the practice of this invention and that the element 22 shown in this illustration is diagrammatic and purely illustrative.

In issuing from the power nozzle 11, the jet entrains fluid from the exist chamber 14 tending to create a low pressure unstable condition therein. The geometry of the exit chamber 14 and particularly of the wall surface 15 on the left is purposely selected to favor the formation of a low pressure bubble tending to draw the jet toward a monostable orientation adjacent the wall 15 to which the jet becomes locked. This attachment of a flud jet to an adjacent non-parallel wall is known as the Coanda effect after its discoverer Henri Coanda. Orifice 12 on the left is a bias port through which a controlled small flow of fluid is admitted as an adjustment of the force of attachment of the jet to surface 15.

Orifice 13 on the other hand is a control port capable when blocked, of forcin-g the power jet to switch from its monostable orientaton along wall 15 to flow along wall 16 and into the feedback channel 21. When orifice 13 is substantially blocked, the entrainment of fluid therefrom creates a low pressure condition on the right which over comes the biased low pressure condition on the left and effects switching of the main jet. Thus when switching ori fice 13 is blocked by occluding member 24 of the vibrator 22, the power jet is diverted into the feedback channel 21. Member 24 is typically placed to the plane of vibrator 22, and is preferably of a length such that when the vibrator is in its neutral or null position, the member 24 covers orifice 13.

The flow of the power jet is thus through the feedback path 21, which directs the jet against the vibrator blade 23 to set the vibrator into oscillation at its resonant frequency. When thus set into vibration, the member 24 of the vibrator alternately covers and uncovers the control port 13, thereby cyclically altering the transverse pressure across the power jet at the point of its emergence from the power nozzle. This causes the fiuid jet to switch from one leg of the oscillator to the other at a frequency equal to the damped natural frequency of the vibrator. The power jet should impinge on the blade of the vibrator at the resonant frequency thereof with a correct phase relationship to excite the vibrator at its point of maximum forced amplitude. When the vibrator is driven at its resonant peak, the amplitude of the harmonics generated within the system can be neglected. The frequency stablity of the device is especially high if the material damping of the vibrator is small, thus giving a sharp resonant peak.

An important relation should exist between the phase of the excitation force and that of the resonator. The oscillator proposed herein is not a linear system and does not lend itself readily to the description of its frequency domain by the usual Laplace transform methods. In FIG- URE 2 is shown a time plot of the instantaneous amplitude of vibration of the vibrating element 22 shown in the previous illustration. T and T represent the time delays that occur between the nominal position of the vibrator at which the oscillator switches and the position :at which the change in jet force is felt at the vibrator blade 23.

At time zero in FIGURE 2, the vibrator is considered to be at its null position with zero defiection and maximum velocty. At this point the vibrator blade 23 is under the influence of the jet stream directed at it through the feedback path 21. At a particular amplitude of vibration X the vibrating element 22 begins to move such that member 24 opens the control port 13 and the oscillator begins to switch the power jet from the feedback branch 21 of the unit to the left exit branch 20. After a small delay the element becomes fully switched with the jet stream directed down the left exit channel 20. However, the trailing edge of the jet stream previously introduced into the feedback path 21 is still in motion and must complete its travel along the feedback path before the force on the vibrator 22 is removed- The total time lag is represented as T Under the influence of the energy already imparted to it the vibrator 22 continues its deflection obtaining its maximum amplitude and then as a result of spring force begins to return in the opposite direction. At approximately the same amplitude level X the member 24 begins to close the control port 13 and the oscillator begins to switch. After a short interval of time sufiicient for the jet stream to entran the necessary amount of gas from the control port 13, the oscillator switches its jet stream back along surface 16 directed toward the feedback path 21. Again, however, the wave front of the fiuid jet must travel the length of the feedback path 21 before the force of the jet stream is applied to the vbrator 22. This total time lag is represented by T which is of course influenced by the length of the feedback path, as well as the velocity of the main jet. The timing of the events in the oscillator is also influenced by the volume of gas contained Within the control port 13 and by the null position of the vbrator 22 relative to the control port opening.

As to structural relationships, it should be noted that the member 24 does not necessarily have to close or occlude control port 13 when the vibrating element is in the neutral or null position, although the device will not be self-starting unless the member 24 is so disposed. The oscillatory member 22 must obviously be spaced so that it does not contact the termination of channel 21 during any part of its operating cycle, and the member 24 should not rub against any part of the element in which channel port 13 is disposed. Further, it should be noted that while member 22 was depicted as a vbrator in FIGURE l, it obviously could be a pendulum without aflecting the basic characteristics of my invention.

FIGURE 2 should be regarded as a schematic representation of the time cycle associated with the embodiment of FIGURE 1, and is shown to better illustrate the sequence of events involved rather than to limit the invention. In fact, under certain conditions, it may, for example, be desired to have the powered portion much shorter than illustrated in FIGURE 2, although the powered portion may or may not still begin at the same time (about t=t Significantly, therefore, time over which the oscillatory member is powered can be changed by properly selecting the aforementioned variables, including jet velocity, and the fluid capacitance in channel 13.

The oscillatory member employed to entrain the frequency of the oscillatory may take many forms in the practice of this invention. A pendulum, or a tensioned band or wire each have the advantage of simplicty. When a tensioned band or wire is stretched in a frame made of the same material, the frequency of vibration is very stable and substantially insensitive to ambient temperature variations except for those which give rise to significant temperature gradients within the structure of the vbrator and its frame. A flat tensioned membrane or a vibratable reed capable, when in vibration, of periodically occluding the control port of the oscillator is also a useful form of vbrator.

One type of vbrator which has been found useful to control lower frequencies of oscillation is shown in FIG- URE 3 to comprise disc 31 of predetermined inertial characteristics fixed to the center of a wire 32, the two ends of which are fixed and held in tension by frame means represented at 33. The disc is capable of vibrating with a rotational oscillation due to the t0rsonal effect of the wire 32. The disc is provided with a cut-out portion 34 positioned to oscillate in front of a control or switching port of the fluidic element, the counterpart of control port 13 of FIGURE 1. The excitation pulse delivered by the feedback channel (the counterpart of port 21) is directed to react against the turbinelike blades 35 shown in the illustration. Thus the forces exerted on the blades 35 are a function of the change in momentum of the gas impinging thereon. This force gives rise to a torque which should act on the blades over a suflicient angle or rotation so that the energy imparted to the wheel in every cycle is equal to the energy lost by the wheel due to the damping in the torsional member 32.

Regardless of the exact form of the vbrator or other oscillatory member used, the amount of energy added to it through the feedback path during each cycle is not greatly material since the energy lost due to damping over one cycle of vibration is a function of the amplitude of the vibration, assuming the period of vibration to be fixed. As the amplitude of vibration increases with the total energy delivered to the vbrator becoming larger, so does the energy lost to damping increase. The only requirement imposed on the amplitude of oscillation is that it should be sufiicient to cover and uncover the control port cyclically. As long as there is sufficient energy in the excitation pulse delivered to the vibrating member to overcome the loss of energy due to damping, the vbrator oscillates in a stable limit cycle, the amplitude of which may vary slightly, but the frequency of which remains essentially constant.

Other modifications and variations in the invention will also occur to those skilled in the art to which this invention pertains. For example, in the examples already discussed, the sole function of the fluid fiowing in the feedback path is to excite the vbrator, which controls the state of the oscillator by merely opening and closing the control port. It is also possible to direct the feedback fluid through the control or switching port and to employ the vbrator to gate the feedback fluid at regular intervals. Thus the oscillator becomes switched by periodic pressure pulses received through the feedback path. In such an arrangement, however, the biased or monostable state of the fluidic element should be such as to direct the fluid jet normally toward the feedback path.

Turning to FIGURE 4, an embodiment illustrative of the broader aspects of my invention is revealed. This component can be made of a number of stacked layers of small size, precisely arranged, and held together by diffusion bonding or the like. In such construction, small etched components may be employed, constructed from suitable material such as copper, brass or stainless steel, with each plano or layer being but a few thousandths inch thick and of a size such as l" on a side.

In this embodiment, gas under pressure is supplied to the matrix 39 containing the fluid passages, the gas being delivered to various chambers 40. Oscillatory member 42 is shown as a cantilevered member fixed at one end to the matrix 39, whereas its other end is supplied with a blade 43 adapted to oscillate in slot 45 as member 42 vibrates. As the vbrator blade 43 moves in the slot, occluding member 44 mounted adjacent the blade periodically interrupts the flow from the nozzles 46 and 47. When the member 44 is not in the path of this flow, the flow from nozzles 46 and 47 cross the slot, and flow into conduits 53 and 63, respectively. Flow proceeds through conduit 53 and serves during most of the cycle of member 42 to deflect the jet of a monostable element 54 away from the preferred leg 55, and to the atmosphere through passage 56 instead. Similarly, flow taking place through conduit 63 serves to deflect the jet of monostable element away from preferred leg 65, and to the atmosphere through passage 66 instead.

In operation, consider the occluding member 44 to be moving tow'ard nozzle 47 as a result of motion of the vibratng element 42. At a certain time, the member 44 interrupts the fluid stream normally entering conduit 63, thus disenabling control port 63 and causing flow of element 64 to exit down the preferred leg 65 and into bistable element 74, to cause it to change from one state to the other. This causes the flow in element 74 to travel down leg 75 at which point the flow is conducted around the feedback p'ath 76 and exits at nozzle 77. This previous sequence had, of course, taken some finite time.

With the proper design, when the flow exits at 77, the vbrator has ceased motion toward nozzle 77, and has just begun to return travel toward the position in which member 44 occludes the flow from nozzle 46. The flow from 77 imparts during this interval a force to the blade 43 sufficient to overcome the loss of energy due to m'aterial damping in vbrator 42, in similar manner as the previously described embodiment of FIGURE l. As the vbrator moves back toward nozzle 46, occluding member 44 soon interrupts the flow from nozzle 46, thus disenabling control port 53 and thereby allowing flow in element 54 to travel down leg 55, thereby switching bistable element 74 from the second state back to the first, such that the flow is out leg 85, through the feedback path 86, and finally impinges on the vbrator blade 43 upon exiting at nozzle S7, to cause the vibratory member again to move toward nozzle '77, and continue in vibratory motion.

It is possible to eliminate either nozzle 77 or 87 and only drive the vibrator from one side. Also, one or both of these nozzles can be placed so as to direct a periodic impinging flow on member 43 from locations between conduits 53 and 63 if for some manufacturing or technical reason such as desirable. In such arrangement, the flow impinges on blade 43 as the vibratory member passes through the neutral position of its periodic cycle. In this manner, a constant force of short duration is asserted against the blade during that period when the blade is travelling at the most constant velocity. This is to say, I may apply the power to the blade over that portion of the trajectory shown in FIGURE 2 at which the slope of the trajectory is relatively constant (about the line X=).

Although I am not to be limited to such details, I may prefer to make the matrix member of several layers comprising: a lower cover plate, to which the flud pressure for the device is supplied; a plane containing passages for this flud pressure; another cover plate containing certain apertures; a plane containing elements 54 and 64; a cover plate containing certain apertures; another plane, containing element 74; another cover plate containing certain other apertures; another plane containing feedback passages 76 and S6; and still another cover plate which forms the outer top cover, to which the vibrator 42 is attached. The outer top cover of course, like the other layers, is equipped with a slot cnabling proper motion of blade 43 therein, and if desired, a cover may be provided over the entire device, although such is not usually utilized.

In connection With FIGURE 4 it should be noted uo that this embodiment is significant in several ways, in that for example it allows one to more easily fabricate, with less close tolerances, the arrangement or position of the occluding member 44 with respect to the control ports 53 and 63. With the mechanization referred to in FIGURE 1, the vibrator must be positioned close enough to the exit of control port 13 such that the occluding member 24 will on occasion disenable the port, and yet not so close that the vibrator will rub against the side of the control port. In practice this would be on the order of of an inch clearance. ln contrast, in FIGURE 4 the plate is simply moved in 'and out of any existing flud streams and therefore need only deflect the flud stream issuing from ports 46 or 47 away from channels 53 or 63 respectively.

I have now shown several preferred embodiments of my mechanically entrained oscillator which utilize a flud control device having first and second output states controlled by the condition of at least one control port. The condition of the control port or ports is in turn determined by the position of an oscillatory member which can take the form of a vibrator, a pendulum, as oscillating wheel, or the like. Furthermore, the oscillatory member is maintained in oscillation at its natural frequency in spite of inherent friction and d'amping by the delivery of flow from one of the control devices to the oscillator member. This flow periodically imparts energy to the oscillatory member to overcome the energy loss, including that due to friction and damping to sustain oscillation at a relatively constant amplitude.

It should therefore be clear that the described embodiments are primarily illustrative, and that the following claims are intended to encompass all variations and modifications as are within the true spirit and scope of the invention in its broader aspects.

I claim:

1. A constant frequency flud oscillator comprising:

a flud control device having first and second output states and including two control ports, the condition of said control ports determining the output state of said device;

a pair of monostable fluidic devices, disposed so that a certain output from each will serve as an input to a respective one or the other of said control ports;

'an oscillatory member positioned to control on an alternating basis, the activation of said monostable devices, and thus controlling by the sequential outputs from said monostable devices, the positioning of said flud control device, and

means responsive to the output state of said flud control device for delivering energy impulses to said oscillatory member to keep the same in oscillation at a stable frequency.

2. A constant frequency flud oscillator comprising:

at least one flud supply nozzle means from which a well-defined stream of flud may exit, an oscillatory member arranged to oscillate between two positions, latter member periodically occluding said nozzle means, a bistable flud element arranged to supply flud selectively to one side or the other of said oscillat'able member so as to keep it in oscillation, a pair of monostable flud elements, each having a preferred output position, the interruption of the flow of flud from the said nozzle means determining the positioning of each of said monostable flud elements, the outputs from said monostable flud elements controlling the positioning of said bistable flud element, thereby to assure the porting of flud in the correct direction to m'aintain a stable frequency for said oscillatory member.

3. A constant frequency flud oscillator comprising:

(a) a vibrating member having a natural frequency of vibraton,

(b) means for applying flud pressure pulses to said vibrating member to sustain vibration at said natural frequency, said means includin a bistable flud element having two output legs, and two control ports for selectively controlling from which leg such pressure pulses flow,

(e) a pair of monostable flud elements, each having a control port and a pair of output legs, one leg of which is preferred, with each of such preferred legs being connected to a respective control port of said bistable element,

(d) nozzle means for supplying a stream of flud, said vibrating member having means mounted thereon for occluding on an alternating basis, the flow of flud from said nozzle means to the control ports of said monostable flud elements,

(e) a given monostable element, when the flow to its control port is interrupted, providing an output along its preferred leg, thus to accomplish the switching of the output of said bistable element from one output leg to the other, and thereby to cause a pressure pulse tending to drive said vibrating member back toward the position in which it occludes the input to the control port of the other monostable flud element,

(f) the interruption of the flow to said monostable elements therefore being on a sequential basis, with the switching of said bistable flud element taking place in response to the timing of such interruptions.

4. A constant frequency flud oscillator comprsing:

a pair of nozzles from which well-developed streams of pressurized flud can on occasion issue, a vibratable occluding element mounted in a generally cantilever fashion and arranged when vibrating to alternately occlude during a portion of its cycle, one or the other of said nozzles, the first of said nozzles, when not occluded, being arranged to direct its flow into the input port of a first monostable flud element, such monostable flud element having two output legs, one of which is preferred, with flow from said preferred leg being shifted to the other leg in the presence of flow from such nozzle, the other of said fluid nozzles, when not occluded, being arranged to direct its flow into the input port of a second monostable fluid element, and upon such occasion biasing the flow of its fluid away from its preferred leg, the flow down such preferred legs of said monostable fluid elements being directed to respective input ports of a bistable fluid element having two input ports and two output legs, the flow from one output leg of said bistable fluid element tending to cause the vibratable occludng element to travel in one direction, and the flow from the other leg of the bistable element causing said vi bratable occludng ele-ment to travel in the opposite direction, the timing of the said oscillator being such that when the first of said nozzles is occluded, the output flow -from said bistable element is such as to move said occludng element in the direction of said other nozzle, and when said other nozzle is occluded, the flow from said bistable element is in the direction to tend to move said bistable element back toward said first nozzle.

References Cited UNITED STATES PATENTS Severson 13781.5 Woodward 13781.5 X Warren 13781.5 Horton et al. 13781.5 Vockroth.

Bauet 13781.5 Spyropoulos 137-81.5 Greenblott et al. 13781.5 X Katz 13781.5 X Boothe 13781.5 X Meier 13781.5

SAMUEL SCOTT, Primary Examiner. 

